This invention relates to acoustical imaging and, more specifically, relates to the production of colorflow images based on the Doppler shift of ultrasonic signals.
Most mid to high end ultrasound systems include a color flow processor which enables a physician to image blood flow in the body as a tool in the diagnosis of cardiac and vascular dysfunction. Lately this has also been applied to imaging tissue motion. To accomplish this task, the ultrasound system sends and receives a series of firings (packets) along each line of sight, and extracts one or more functional parameters from the return signals using an estimation algorithm. These parameters are then mapped into various colors and displayed as an image overlaying a B mode anatomical image. Traditional parameters extracted are mean blood flow velocity, flow variance, and the strength of the flow signal, or flow power.
Almost all commercial ultrasound systems use an autocorrelation algorithm to extract flow information, since it is a relatively simple, robust algorithm for calculating the mean flow velocity and a crude form of variance can be extracted from its terms. But being a Doppler based estimator, it suffers the same deficiencies as other Doppler based processes, i.e., the reliability of the velocity estimate is inversely linked to the spatial resolution in the image. This deficiency has led investigators to develop a new class of time domain (wideband) estimators having velocity estimate resolution which relies on good spatial resolution. These estimators, however, tend to suffer from signal to noise ratio problems, and are best in high signal noise ratio (SNR) environments.
The autocorrelation algorithm used in most commercial ultrasound systems is useful for estimating material movement characteristics, such as blood flow, in a variety of different portions of a subject under study, such as portions of the human body. However, there are other algorithms which offer advantages for certain portions and certain types of examination. It would be advantageous to have a color flow imaging system which could utilize the best available algorithm for imaging a particular portion of a subject under study and for imaging in connection with a particular type of examination. This invention solves that problem.